Question

I’d like to know whether having different variables for the src (source) and dst (destination) of an OpenCV function will have an effect on the processing time. I have two functions below
that does the same thing.

public static Mat getY(Mat m){
    Mat mMattemp = new Mat();
    Imgproc.cvtColor(m,mMattemp,Imgproc.COLOR_YUV420sp2RGB);
    Imgproc.cvtColor(mMattemp,mMattemp, Imgproc.COLOR_RGB2HSV);
    Core.inRange(mMattemp, new Scalar(20, 100, 100), new Scalar(30, 255, 255), mMattemp);
    return mMattemp;
}

VERSUS

public static Mat getY(Mat m){
    Mat mMattemp_rgb = new Mat();
    Mat mMattemp_hsv = new Mat();
    Mat mMattemp_ir = new Mat();
    Imgproc.cvtColor(m,mMattemp_rgb,Imgproc.COLOR_YUV420sp2RGB);
    Imgproc.cvtColor(mMattemp_rgb,mMattemp_hsv, Imgproc.COLOR_RGB2HSV);
    Core.inRange(mMattemp_hsv, new Scalar(20, 100, 100), new Scalar(30, 255, 255), mMattemp_ir);
    return mMattemp_ir;
}

Which of the two is better? What is the advantage of one over the other?

Answer 1

To know for sure, just sandwich your getY method calls between OpenCV’s built-in methods double getTickCount() and double getTickFrequency() like this (will need to translate to java):

//first uniquely name the algorithms to compare (here just getY_TypeA and getY_TypeB)
double durationA = cv::getTickCount();

getY_TypeA(image); // the function to be tested

durationA = cv::getTickCount()-durationA;
durationA /= cv::getTickFrequency(); // the elapsed time in ms

//now call the other method

double durationB = cv::getTickCount();

getY_TypeB(image); // the function to be tested

durationB = cv::getTickCount()-durationB;
durationB /= cv::getTickFrequency(); // the elapsed time in ms

printf("Type A runtime: "+durationA+" Type B runtime: "+durationB);

For best results do this for multiple calls and average the results.